Piston with central valve for hydraulic brake systems

ABSTRACT

The present invention relates to a piston with a central valve, in particular for hydraulic boosters, the valve case (6) which is composed of a supporting insert (8) radially embraced by a valve seat (7). It is disadvantageous in some valve cases that sometimes the positive coupling between the two elements in axial direction is not sufficient. The present invention provides a remedy to this problem by using undercuts behind which the valve seat engages the supporting insert in both directions. In an advantageous improvement, the valve insert is of symmetrical design, and a sealing projection is shaped at the valve seat. The guidance of the valve tappet (19) is performed within the piston (1).

BACKGROUND OF THE INVENTION

The present invention generally relates to a piston for hydraulicvehicle brake systems. Pistons for such brake systems are known, forexample, from DE-OS 39 32 248.

It is considered disadvantageous in conventional central valves thatspecial care must be taken when mounting the valve case; the combinationof the valve seat and supporting insert. If the valve pin is notparticularly reliably coupled to the supporting insert, e.g. byvulcanization or cementing, the valve seat may become detached from thesupporting insert due to the lateral friction at the receiving bore inthe piston when the entire valve case is fitted. This is because onlythe forces which result from the frictional engagement or the adherenceeffect of the adhesive, respectively, can counteract an axialdisplacement of the two inserts.

Therefore, the present invention provides a piston for hydraulic vehiclebrake systems and its object is to further simplify this piston byaffording, in particular, greater ease of manufacture and assembly.

SUMMARY OF THE INVENTION

This object is achieved by providing a piston having two end surfaces onan annular collar enclosed by a valve seat within the piston. Hence, inprinciple, the present invention resides in providing the combination ofa valve seat and supporting insert with axially defined undercutsadapted to receive, without difficulties, the axial forces which developduring the assembly and also during the operation so that the valve seatcannot move axially relative to the supporting insert.

In the prior-known valve case according to DE-OS 39 32 248 thesupporting insert extends with an annular end into a stepped bore of thepiston. Therefore, in order to be able to fit the valve case,spiral-shaped venting grooves are necessary, and a double centeringresults relative to the valve case which is usually undesirable.Further, the exact manufacturing tolerances necessary for this purposeare difficult to achieve.

Further, special attention must be paid, in the case of automatedassembly, that the valve case is not fitted in an axially wrongposition. This invention affords further simplifications by providingthe combination of a collar having end surfaces enclosed by the valveseat with the valve case symmetrically disposed about a median plane ofthe collar which extends transversely to an axis through a central duct.In other words, this implies that one manages without the annularprojection at the supporting insert and, hence, without a continuousguidance of the valve tappet within this supporting insert. Therefore,the valve case can be mounted in both axial positions. In addition, thesealing effect of the valve case relative to the piston bore is stillaugmented, because the two-sided sealing surface and the end surface ofthe valve case contribute to the sealing relative to the piston bore atthe bottom of this bore. Further, the valve case is easier tomanufacture because of the measure mentioned above, since the positionof the supporting insert during the coupling with the valve seat isunimportant.

The valve member in the known central valve according to DE-OS 39 32 248is provided with an annular projection in order to enhance the sealingeffect. It may be disadvantageous in such an arrangement that thesealing seat of the valve seat will possibly be damaged by the rigidedges of the valve member. It is considered another disadvantage thatthe annular projection must be manufactured very accurately such thatfinishing usually is necessary. Admittedly, this could be remedied byparticularly precise casting processes, such as aluminum die casting,but manufacturing costs are high for such processes. As a remedy, thepresent invention provides an annular projection on the valve seatdisposed toward the valve member. An augmented sealing effect isachieved by casting an annular projection on the valve seat. Thismeasure permits constructing the valve member in a simple fashion as adrop-forged steel element and removes the need for a casting process.

When the valve case is of symmetrical design, it is advisable to applyan annular receptacle on the bottom of a stepped bore that accomodatesthe valve case for receiving the annular projection in one embodiment ofthis invention. The annular accommodating recess provides an improvedaccuracy of the valve case size relative to the piston because each ofthe two end surfaces of the valve seat is provided with an annularprojection. Further, possible axial forces which might emanate from theannular projection pressed in on the bottom of the bore are kept awayfrom the valve case.

When casting on the valve seat at the supporting insert, the latter hasto be fixed in a mold beforehand. It is preferable to provide an axialbore of the supporting insert with annular recesses at its ends in orderto avoid damaging the axial bore of the supporting insert.

In order to obtain the inventive undercut between the valve seat andsupporting insert, a large number of shapings of the supporting insertare possible. The combination including a rectangular cross-section onthe annular collar provides a particularly simple and reliably actingshaping. A supporting insert of such shape can be obtained particularlyeasily by a casting process or a pressing process.

Another embodiment having the supporting insert achieves augmentedstrength, especially in an axial direction by including a conical taperon the supporting insert.

It has already been explained hereinabove that the known valve caseaccording to DE-OS 39 32 248 principally necessitates double centering.On the other hand, this valve case is desired to be used at the sametime for the true-to-size guidance of the valve tappet. When applyingthis principle to a valve seat in hydraulic brake systems, the tappetwould have to extend through two bores which align with each other. Suchalignment might entail undesirable difficulties which are eliminated inan improvement provided by the present invention. Namely, the diameterof the axial bore of the valve case is larger than the diameter of thesubsequent axial bore of the central duct in the piston. Hence, thevalve tappet is guided only in the axial bore of the piston so that thewalls of the axial duct simultaneously serve as guiding surfaces.

As a material combination, it has proven expedient to use aluminum forthe supporting insert and for the piston itself. The structure of theindividual component parts in relation to each other has been chosensuch that no adverse effects result from the valve member being made ofa different material; preferably steel. As an auxiliary measure, it maybe advisable to insulate the surfaces of different metals which slide oneach other by coating them, which can be effected for instance bynickel-plating, by zinc-plating, or similar coating methods.

Pistons according to the present invention are required in master brakecylinders with a central valve for anti-lock hydraulic brake systems(ABS). There is the difficulty in these brake systems that the centralvalve has to be opened under pressure. From this ensues the risk thatthe valve seat, which is composed of a relatively soft elastic material,is damaged by the flow forces developing at the opening valve. Theproblems existing in this respect have been described in detail in DE-OS40 40 271. Further, a solution has been indicated in DE-OS 40 32 873arranging for two valves to be connected in series. Another suggestion(see, for example, P 42 23 353) made in this respect is directed toreplacing the function of the second valve, which protects the centralvalve, by a simple restrictor or a restrictor controllable bydifferential pressure.

Another object of the present invention is to improve upon the design ofthe central valve including the associated protection devices describedhereinabove regarding the reliability and the speed of the mode ofoperation of the central valve and to better protect the central valveon opening against the high pressure in the working chamber. To thisend, the central valve according to the species can be furnished withthe initially described features respectively or in combination.

In this embodiment, this object is achieved by the combination of aprotection device connected upstream of the central valve in thedirection from a working chamber at the open piston end to the centralduct. That is to say, in principle, the present invention provides acontrolling protection device for the central valve according to theembodiment which limits the quantity of pressure fluid which maximallyflows through the central valve, in particular when pressure prevails inthe working cylinder.

According to favorable improvements provided by the present invention,the controlling protection device can be a stationary restrictor, or adynamic restrictor. Details in this respect are explained more closelyin P 42 23 353. P 42 23 353 is hereby incorporated by reference intothis application.

Another advantageous embodiment of this invention includes an upstreamconnected protection valve like that explained in principle in DE-OS 4032 873, for example.

Two embodiments of the present invention are described hereinbelow withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional view of a piston according to thisinvention with a central valve.

FIG. 2 is a partial sectional view of a piston according to thisinvention with an additional protection valve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a piston according to the present invention will be describedwith reference to FIG. 1. DE-OS 39 32 248 discloses individual elementsof a master cylinder, in particular a tandem master cylinder, which canutilize the piston of the present invention.

The piston 1 shown in a cross-sectional view, which can be, for example,e.g. the push rod piston or the secondary piston of a tandem mastercylinder, is furnished with a stepped bore 2 which consists of a firstbore section 3 of large diameter and a stepped second bore section 4 ofslightly reduced diameter and a third bore section or central duct 5 ofa relatively greatly reduced diameter.

The drawing shows a valve case 6 which includes a valve seat 7 and asupporting insert 8. The valve seat 7 is vulcanized or cast on thesupporting insert 8. In order to improve the receiving of axial forcesbetween valve seat 7 and supporting insert 8, the supporting insert 8 isfurnished with an annular collar 9A, 9B. The drawing shows two versionsof an annular collar which can be used optionally. On the right-handside of the drawing a collar 9A with rectangular cross-section is shownwhich is used preferably. Another possibility resides in furnishing thecollar 9B, as is depicted on the left-hand side of the drawing, with asubstantially conical shape which passes over into a rectangularsection. In both versions, the annular collar 9A, 9B is enclosed on bothof two end surfaces 69, 70 by the valve seat 7.

The valve insert 6 is preferably symmetrical relative to its medianplane that lies transversely to the longitudinal axis of the bore 3 sothat it can be mounted in both positions. The diameter C of the axialbore 10 of the supporting insert B is larger than the diameter D of thecentral duct 5. At both ends of axial bore 10, supporting insert 8 isprovided with two annular recesses 11 and 12 which are meant to prevent,in the manufacture of the valve case 6, tapering of the ends of bore 10during the manufacturing process.

At least at one of its two end surfaces 15, 16, the valve seat 7 carriesan annular projection 13 which serves as a means for improving sealingin relation to a valve member 17. On the right-hand side of the drawing,a symmetrical design of the annular projection is illustrated so thatthe valve seat 7 in this case comprises the annular projections 13 and14. It is preferable that the piston 1 is provided with a mating annularreceptacle 18 on the bottom of the second bore section 4, in order toprovide a smooth abutment surface on the bottom of the bore section forthe end surface 16. In an alternative embodiment, annular receptacle 18is eliminated (according to the left-hand side of the drawing). It isadvantageous in this latter embodiment that valve seat 7 comprises anannular projection 13 on only one side. In this case, there is notcomplete symmetry of the valve case 6.

The valve member 17 is furnished with a tappet 19. The diameter of theaxial bore 10 is chosen to be larger than the diameter of the centralduct 5 such that the tappet 19 is guided on the peripheral surface ofthe central duct 5. Since the duct 5 serves simultaneously as apressure-fluid duct, the tappet 19 takes support on the bore wallspreferably only by means of three webs 20. The webs 20 extend in thelongitudinal direction of the axis of the stepped bore 2 and are evenlydistributed on the periphery of the tappet.

The valve member 17 is biased relative to the valve seat 7 by means of aspiral spring 21. Spring 21 bears against the piston by means of a propring 22 which is adhered to the piston 1 or is permanently fixed in anyother conventional fashion. However, the prop ring 22 can also snap in acorresponding receiving groove in order to simplify the structure of thecentral valve.

Piston 1 and the supporting insert 8 are preferably made of aluminum,while the valve member 17 is pressed from steel. (e.g. forged ordrop-forged).

An embodiment of the present invention with a protection device will beexplained in the following with reference to FIG. 2. Regarding thecentral valve, reference is made to the description of FIG. 1, andregarding the design of the protection valve, reference is made to DE-OS40 32 873 which is hereby incorporated into this specification byreference.

The master brake cylinder 67 is composed of a housing 68 with ablind-end bore 63 in which a piston 1 is sealingly guided.

A spring 60 is interposed between a non-illustrated secondary piston andthe piston 1.

A central valve 6, 38 is arranged in the piston 1. A more precisedescription of the central valve will follow below.

A tappet 19 extends in the direction of the longitudinal axis of thepiston 1 and serves to actuate the central valve 6, 38, as will beexplained in the following. The tappet 19 abuts transverse pin 59, whichis illustrated in part only, and keeps the central valve 6, 38 open.

The piston 1 confines a working chamber 62 which is connected with thebrake circuits and with the outlet of the pump.

The tappet 19 is guided in a narrow longitudinal bore which servessimultaneously for the exchange of pressure fluid between the workingchamber 62, and a supply reservoir, respectively. In order that anunhindered pressure fluid balance occurs, a compensating bore 23 can beprovided in the piston 1 which extends substantially transversely to thelongitudinal axis of the piston. As a substitute, the tappet 19 can beprovided with longitudinal grooves.

The piston 1 is provided with a longitudinal bore 31, stepped severaltimes, which extends from the frontal end confining the working chamber62 up to the compensating bore 23. The largest step of the longitudinalbore is disposed adjacent the frontal end of the working chamber 62.This portion ends at a step 32 on which a cylinder-shaped insert member33 is supported. The insert member 33 is provided with a seal 34 sealingthe insert member at the outside periphery, thereby permitting merelypressure fluid flow through the central duct of the insert member 33.Due to a special outside contour, the insert member 33 can also bepressed sealingly into the bore. The central duct is provided with atempered valve seat 35 on its end close to the working chamber 62.Beginning at the insert member 33, another portion of the longitudinalbore 31 extends to a second step 36 which has an annular surface.Following the second step 36, finally, is a duct 5 in which theactuating tappet 19 is guided. Also, the compensating bore 23 branchesoff from the duct 5, allowing the duct 5 to be very narrow, at least inpartial areas, so that it may serve as a guidance for the tappet 19.

Arranged in the space between the first step 32 and the second step 36is a closing member 38 at one end of the tappet 19. The diameter of theclosing member 38 corresponds substantially to the diameter of thesection of the longitudinal bore 31 that receives closing member 38,thereby providing a guidance. Pressure-fluid flow past the closingmember 38 is realized by passages 39 defined near the outer rim of theclosing member 38.

As a movable seat, the central valve has the closing member 38(preferably made of steel) which is inserted between an actuating pin 43and the tappet 19. This movable closing member 38 cooperates with avalve case 6 which is inserted in the second step 36 of the longitudinalbore 31. The stationary valve case 6 substantially consists of anannular valve seat 7 which is mounted on a supporting insert 8. Furtherdetails in this regard are explained in reference to FIG. 1 above.Simultaneously, the supporting insert 8 forms a stop for the movableclosing member 38. A bead 53 which extends annularly in the direction ofthe closing member 38 takes care of the necessary sealing of the centralvalve 6, 38 in the closed condition. The distance between the bead 53and the closing surface of the closing member 38 is referred to by B.

The actuating pin 43 projects through the central duct of the insertmember 33 and abuts on a valve ball 44 which cooperates with the valveseat 35. The valve ball 44 is held by a guide element 45 on which aspring 46 is supported. On its other side, this spring 46 bears againsta bushing 47 which is installed on the piston 1. It must be noted that aspring 49 is supported between the insert member 33 and the closingmember 38 which, like the spring 46, ensures the reliable closing of therespective valve.

The connection between the valve closing member 38 and the actuating pin43 is adapted such that, in the basic position of the central valve, thefirst valve formed of valve ball 44 and valve seat 35 has a firstopening distance A, while the second valve formed of valve seat 7 or thebead 53, respectively, and the closing member 38 has a second openingdistance B. The first opening distance A is preferably smaller than thesecond opening distance B.

The piston 1 of the master brake cylinder operates as follows. FIG. 2shows a non-actuated condition of the master brake cylinder; the centralvalve is in its open condition. The piston 1 adopts its basic position,the tappet 19 abuts on the associated transverse pin 59 so that thevalve seat 35 designed on the piston 1 is spaced from the valve ball 44.A pressure fluid connection is established between the working chamber62 and the supply reservoir. Once the brake pedal is depressed, the pushrod piston 1 displaces under the effect of the pedal force to the leftin the drawing and entrains a non-illustrated secondary piston via thespring 60. The pistons are displaced relative to the housing 68, whilethe tappet 11, the closing member 38, the actuating pin 43 and the valveball 44 are not displaced relative to the housing of the master brakecylinder. The springs retain the valve members in their position. Assoon as the piston 1 has covered the first opening distance A, the valveseat 35 will move to abut on the valve ball 44 so that theabove-mentioned connection between the working chamber 62, on the oneside, and the supply reservoir, on the other side, is interrupted. Whenthe piston continues to displace, pressure can now develop in theworking chamber 62.

When the piston 1 is moved further to the left, the actuating pin 43will move away from the valve ball 44, and the valve seat 38 approachesthe bead 53. As soon as the piston 1 has covered the second openingdistance B, the central valve 6, 38 will close as well. This is effectedwithout pressure, since no pressure is built up in the space between thetwo individual valves. Thus, the valve seat 7 which consists of rubberor any other suitable sealing material is not damaged.

Provided that the first protection valve 44, 35 closes reliably, thebehavior of the central valve 6, 38 is of no consequence.

In a control operation when pressure fluid is supplied by the pump intothe working chamber 62, the piston 1 slides back into its basicposition. Then, central valve 6, 38 opens first, and next the protectionvalve 44, 35 opens. Now pressure fluid can flow past the openingprotection valve 44, 35 and past the open central valve 6, 38--theopening gap distance is equal to B minus A. Because of the protectionvalve 44, 35 which is still closed at first, the quantity of pressurefluid discharging through the central valve 6, 38 is limited (orrestricted) so that destruction of the rubber seal (valve seat 7) isavoided. The minor pressure reduction occurring in the working chamber62 results in the working piston 1 being shifted to the left (accordingto the drawing) again and the protection valve 44, 35 closing.Consequently, a control operation is performed which consists of rapidlysuccessive opening and closing actions of the protection valve 44, 35.Eventually, as much pressure fluid as is supplied by the pump into themaster cylinder can discharge through the valve. The master cylinderpressure corresponds to the pedal pressure.

The preceding description is exemplary rather than limiting in nature.The described embodiments could be modified or varied without departingfrom the purview, scope and spirit of this invention which is limitedonly by the appended claims.

What is claimed is:
 1. A piston with a central valve, a cavity formed insaid piston and open at a first piston end, and a central duct formed insaid piston between said cavity and a second piston end for use inhydraulic vehicle brake systems, comprising:a valve disposed within saidpiston adjacent a first end of said central duct having a rigidsupporting insert and an annular elastic valve seat which abuts at leaston a peripheral surface of an annular collar of said supporting insert,said collar having two end surfaces enclosed by said valve seat whereinsaid valve seat includes an annular projection disposed on one end ofsaid valve seat toward said central valve.
 2. A piston as claimed inclaim 1, wherein said valve case is designed symmetrically relative to amedian plane of said collar which extends transversely to the axis ofsaid central duct.
 3. A piston as claimed in claim 2, wherein saidcavity includes an annular receptacle for receiving an annularprojection disposed on one end of said valve seat toward said centralduct.
 4. A piston as claimed in claim 1, wherein said supporting insertincludes an axial bore having two annular recesses.
 5. A piston asclaimed in claim 1, wherein said annular collar has a rectangularcross-section.
 6. A piston as claimed in claim 1, wherein a peripheralsurface on said supporting insert tapers conically towards said collarfrom end surfaces defined on said support insert.
 7. A piston as claimedin claim 1, wherein said valve case has an axial bore with a diameterthat is larger than the diameter of said central duct.
 8. A piston asclaimed in claim 1, wherein said supporting insert is made of aluminum,said central valve is made of steel and said piston is made of aluminum.9. A piston as claimed in claim 1, further comprising a protectiondevice for governing the quantity of pressure fluid flowing through saidcentral valve, said protection device connected upstream of said centralvalve in the direction from a working chamber at said open piston end tosaid central duct.
 10. A piston as claimed in claim 9, wherein saidprotection device comprises a restrictor.
 11. A piston as claimed inclaim 10, wherein said restrictor is a valve.
 12. A piston as claimed inclaim 9, wherein said controlling device comprises a protection valveconnected upstream of said central valve.
 13. A piston as claimed inclaim 12, wherein said protection valve comprises a valve seat and avalve closure member made of metal, respectively, and wherein saidcentral valve comprises a valve seat and a valve member, said valve seatcomprising a seal made of a soft-elastic material.
 14. A piston asclaimed in claim 13, wherein said valve member of said protection valvecomprises a valve ball, and wherein said closing member of said centralvalve comprises a steel cylinder.
 15. A piston as claimed in claim 12,wherein said central valve and said protection valve are designed insaid piston, and said valve seats of each said valve are designed insaid piston.
 16. A piston as claimed in claim 15, wherein said valveseat of said protection valve is provided on an insert member which isrigidly coupled to said piston.
 17. A piston as claimed in claim 12,further comprising a transverse pin inserted to extend through ablind-end bore of a master brake cylinder, and an actuating tappet onsaid pin for actuating said protection valve and said central valve ismovable into abutment.
 18. A piston as claimed in claim 17, furthercomprising a closing member on said central valve on said actuatingtappet whereat said actuating tappet is adapted to be movable intoabutment on said transverse pin.
 19. A piston as claimed in claim 18,wherein said closing member is connected to said protection valve by wayof an actuating pin for said protection valve.
 20. A piston as claimedin claim 12, wherein a basic position of said central valve includessaid valve member of said central valve having a first distance fromsaid central valve seat and said valve member of said projection valvehas a second distance from said projection valve seat, said firstdistance being smaller than said second distance.